1. Field of the Invention
The present invention relates to a method utilizing bending moment measurements taken near a drill bit to control the direction of the drill bit.
2. Description of the Prior Art
When a borehole is drilled in the search for petroleum, it is necessary to control the direction of the drill bit in order to intercept the target formation at the correct location. It is substantially impossible to drill a straight borehole utilizing either of the two standard techniques, namely either by rotating the entire drill string by a kelly and associated equipment or by having a downhole motor driving the drill bit. Many factors, such as gravitational effects, bending of the drill collars, the torque of the drilling action and the anisotropic failure character of rock contribute to causing deviation in the drill bit direction.
In the past, the one way of controlling the drill bit direction has been to cease the drilling operations and lower into the borehole an instrument to make directional measurements. These measurements would then be compiled with a past performance record and a model for the desired borehole path and the desired corrections made by adjusting the weight-on-bit, rotary speed, etc. to achieve the desired change in direction of the bit. If these changes proved inadequate to redirect the bit to the desired direction, it was necessary to pull the drill string from the thole and change the down hole assembly configuration of the drill string. It will be appreciated by those skilled in the art that any operation, such as this, which interrupts drilling is going to be costly and time consuming.
Another more recent development is measuring-while-drilling equipment which enables directional measurements to be made and corrections accomplished without interrupting the drilling operation. However, the hole must be drilled and surveyed by the measuring-while-drilling equipment before one can determine the result. An example of such a method may be found in U.S. Pat. No. 4,324,297 in which strain measurements are taken immediately adjacent the drill bit. The inclination and orientation of the drill bit are measured, along with the weight on the drill bit, and all of these measurements transmitted to the surface, preferably by a high data rate hard wired electronic system, where they are compared to a model developed from the previous measurements to determine the deviation of the drill bit from a desired course. The weight-on-bit and rotary speed are then controlled to achieve a correction to return the bit to the desired path. While this is an improvement over stopping the drilling for measurement, as previously discussed, it still has a number of drawbacks. These include the amount of data which must be transmitted to the surface, only a portion of which is subsequently used, and the cost and trouble of deploying a hardwire system.